Recently, attention has been drawn to a PRAM (Phase-change Random Access Memory) for storing information therein by using a phase-change film as a high-speed nonvolatile memory having a long lifetime. The phase-changing film is made of a material that becomes amorphous state with a high resistance value by being rapidly cooled after being heated to a high temperature (e.g., about 600° C. or higher), and also becomes crystalline with an ordinary resistance value by being gradually cooled after being heated to a relatively low temperature (e.g., about 400° C. or higher). The PRAM stores data therein by using a difference between the resistance values of these two phases. Such a phase change may be made by controlling a magnitude of a current pulse. That is, by flowing a current having a large pulse, the amorphous state may be obtained, whereas by flowing a current having a small pulse, the crystalline state may be obtained.
Ge2Sb2Te5 serving as a Ge—Sb—Te film, has been used as a material for forming such a phase-change film in the PRAM. Generally, the Ge—Sb—Te film is formed by a PVD method such as sputtering. Since, however, a sufficient step coverage cannot be obtained by the PVD method, it has been attempted to form the phase-change film by a CVD method in order to obtain a good step coverage.
However, it has been found out that a Ge—Sb—Te film may have a poor surface smoothness when the Ge—Sb—Te film is formed by the CVD method using a Ge compound, a Sb compound, and a Te compound as source materials for the film formation.